Recently, great expectations have been placed on an electric vehicle consuming less energy in travelling and a power generation system using natural energy like solar light and wind power in view of preventing global warming and concerns for depletion of fossil fuels. However, the above technologies have the following technological disadvantages, which prevents wide spread of those technologies.
An electric vehicle is provided with a driving battery with a low charge/discharge capacity, and therefore tends to have a shorter travel distance than a typical vehicle per one-time charge. Meanwhile, a power generation system using natural energy is provided with a large capacity of battery for leveling outputs from largely fluctuated power generation, likely resulting in a factor of high costs. Thus, as for any of the above technologies, demanded is an inexpensive secondary battery having a high charge/discharge capacity to solve the above disadvantages.
For example, a lithium ion secondary battery has a charge/discharge capacity higher than secondary batteries such as a nickel-hydrogen battery and a lead battery. Therefore, a lithium ion secondary battery is expected to be applied to a battery for driving an electric vehicle and a power generation system. However, a more greatly improved lithium ion secondary battery having a further increased charge/discharge capacity is strongly desired to meet the demands on a battery for driving an electric vehicle and a power generation system. For satisfying those demands, a cathode and an anode of a lithium ion secondary battery are preferably to have more greatly increased charge/discharge capacities.
As for a cathode active material included in a cathode of a lithium ion secondary battery, a raw material having a layered structure assigned to R3-m and represented by the composite formula of LiMO2 (i.e., a layered structural compound, where N is a metal element other than Li) has been widely used. In the raw material, when Ni is largely contained as a metal element H, a higher content of Ni tends to improve a battery capacity of a lithium ion secondary battery. Particularly, when a content of Ni in the metal element M is more than 70 atom %, a high reversible capacity of more than 180 Ah/kg can be achieved, allowing improvement of the charge/discharge capacity per mass.
On the other hand, realization of such a high reversible capacity causes a large volume change in a crystal structure at charge/discharge operation, by which particles of a cathode active material cause cracks associated with charge/discharge cycles. Accordingly, particle resistance increases (i.e., conductivity decreases) so that DC resistance of a lithium ion secondary battery increases. That is, internal resistance of battery increases associated with charge/discharge cycles. Hence, it is preferable to prevent such an increase in the internal resistance in order to achieve an excellent cycle property.
Here, a technology described in Patent Document 1 is known related to a technology for realizing both highly improved outputs and longer duration. Patent Document 1 discloses a cathode used for a lithium ion secondary battery that includes a compound as an active material capable of intercalating/de-intercalating lithium ions. Herein, the cathode contains a conductive agent and a binder. When a specific surface area of an active material is A (m2/g), a content of the active material in the electrode is B mass %, a specific surface area of the conductive material is C (m2/g), a content of the conductive material in the electrode is D mass %, and an exposed ratio of the active material of a cathode bending cross-section is E (%), a cathode used for a lithium ion secondary battery satisfying the following equation (1) is described.E<50×(A×B+0.05×C×D)/(A×B+0.1×C×D)  Equation (1)